The constantly meshed type multi-stage transmission has a configuration in which one of the drive gears and the driven gears are fixed on a gear shaft, while the other of the drive gears and the driven gears are rotatably supported on another gear shaft, and in which one of the rotatably supported gears is selected to be engaged with the other gear shaft by an engaging means, to make a gear shift.
Usually, the driven gears are rotatably supported on the other gear shaft, and, for the engagement of the driven gear with the other gear shaft, that is, the driven gear shaft, a variety of engaging means have been proposed (refer to Patent Document 1, for example).    [Patent Document 1]    Japanese Patent Publication No. 45-35687
The engaging means used in the constantly meshed type multi-stage transmission disclosed in Patent Document 1 has such a structure that a tubular body is provided which is formed with axially extending grooves each having a widely cut hole widely opened in the circumferential direction at a middle portion of each groove, the tubular body is externally fitted over a driven gear shaft so as to be movable in the axial direction, and a plurality of driven gears are rotatably supported on the tubular body.
The driven gear shaft has in the outer surface thereof flat cutouts at axial portions corresponding to the driven gears. Rollers are put respectively on each of the cutouts of the driven gear shaft, and are loosely fitted in the grooves and the widely cut holes of the tubular body, the rollers in each of the grooves being arranged in a row.
In the above structure having the driven gears rotated while being constantly meshed with the drive gears, the rollers in the grooves do not make contact with the inner peripheral surfaces of the driven gears, so that rotation is not transmitted to these driven gears, and only the rollers in the widely cut holes make contact with and are engaged with the inner peripheral surface of the relevant driven gear, so that rotation is transmitted to the relevant driven gear.
In other words, the driven gear corresponding to a position where the widely cut holes are located transmits the rotation to the driven gear shaft, so that gear shifting operation can be achieved by moving the tubular body in the axial directions.